This invention relates to flame monitoring of boilers with tangential firing, and particularly to a scanner system for monitoring the flames of the individual burners in the tangentially fired boiler.
Tangentially fired boilers are characterized by a combustion chamber which is generally square in horizontal cross-section. The combustion chamber is enclosed by four walls lined with collant-carrying tubes, and the boiler is provided with burners at each corner which fire into a large central fireball. Generally, several different levels or tiers of burners are provided and different fuels are often burned on each level. A liquid heat transfer medium, often water, is circulated through the tubes to remove heat from the furnace. As a secondary effect the medium-carrying tubes sheild the boiler casing to prevent heat damage.
It is desirable to observe the flame of each burner in the boiler to assure that each burner is operating as intended. In the past there have been attempts to employ a flame scanner which sights through the burner wind box at each corner where a small narrow angle opening into the boiler is provided. A scanner mounting system of this type limits the ability of the scanner to see the burner flame. Scanners used in the past employ extended sensing tubes which position the sensor at the furnace end of the burner support and tilting structure (bucket), and employ a flexible mounting assembly in order to allow the sensing tube to tilt with the burner. These sensors required extreme methods of cooling (air or water) in order to maintain the tube temperature below 400.degree. F. The position of the sensor with regard to the burner tip and within the space provisions of the "bucket" caused the sensor to detect flame through the unburned fuel skirt of the burner and could cause the sensor to respond to the radiation from the fireball rather than solely from the associated burner. As a consequence, unignited fuel can be introduced into the furnace resulting in explosions within the furnace under light-off cold furnace conditions.
It has been suggested that scanners be mounted above the burner pods in order to look down on the fireball. However, such a system would not be able to detect a flame-out condition in a tiered, multiple burner boiler, since the scanner would lock onto the fireball and fail to detect a problem with an individual burner within the boiler.